DE69131278T2 - BASIC FILM FOR RETRORE-REFLECTIVE ARTICLES AND METHOD FOR THEIR PRODUCTION - Google Patents

Description

Field of the invention

The invention relates to a new carrier film and retroreflective articles which are used, for example, as transparent coatings for protecting documents from tampering, and includes the same. The invention also relates to a method for producing such carrier films and retroreflective articles.

Background of the invention

Documents often have adherent, transparent overlays to protect against dirt, moisture, and general wear and tear. A typical transparent overlay comprises a plastic film bearing an aggressive adhesive layer that allows it to be permanently bonded to the front of a document. A transparent overlay that does not obscure the underlying information is disclosed in U.S. Patent No. 3,801,183 (Sevelin et al.). The patent discloses inscription-containing sheet materials having retroreflective inscription and retroreflective background areas that are substantially undetectable under conditions of viewing under ordinary diffuse light, but clearly visible under conditions of viewing under retroreflective light. Attempts to tamper with the information to which the overlay is applied result in easily detectable destruction of the overlay.

Following US Patent No. 3,801,183, a series of patents were issued disclosing further transparent coatings, each of which can be imaged with a pattern or inscription that can only be seen without further teres when viewed under specular reflection and can be adhered to a document without obscuring the front of the document. See, for example, U.S. Patent No. 4,099,838 (Cook et al.), which discloses an overlay in which the background and image areas of the pattern reflect in contrasting colors. See also U.S. Patent Nos. 4,688,894 (Hockert) and 4,691,993 (Porter et al.), which disclose transparent overlays which function similarly to that in U.S. Patent No. 3,801,183 and which further have the ability to create an authenticating message in the overlay after it has been adhered to a document.

The transparent protective coatings in each of the above-mentioned patents each comprise a monolayer of glass microspheres, which makes the coating expensive and difficult to counterfeit.

U.S. Patent No. 4,869,946 (Clay) discloses a security card comprising a transparent top layer having narrow parallel lenses on its outside and an image-containing carrier film on its inside. The two layers form a lens-like system that allows images on the carrier film to be selectively visible depending on the angle at which the card is viewed.

In U.S. Patent No. 3,503,315 (de Montebello) a lenticular plate or sheet is disclosed comprising adjacent small lenses with spherical convex surfaces on both sides. The sheet is used to produce and visualize an image in a panoramic stereoscopic relief.

US Patent No. 2,951,419 (Lemelson) discloses retroreflective devices that appear to an observer moving with a light source as illuminating and extinguishing appearing dazzling, sparkling or changing their optical composition. In one embodiment, the device comprises a transparent sheet and a retroreflector which meet at an interface. At the interface there are two groups of adjacent bands or stripes of different color or different light reflectance. On its front side the sheet is provided with a number of lens-like formations. An observer moving with a light source sees alternating color patterns because the discrete bands of light from the moving light source enter the sheet in such a way that the bands change from one group to the other group and vice versa.

AT-A-332,763 discloses a retroreflective sheeting having an array of convex microlens surfaces on both the front and back surfaces of the sheeting, the back surface also being reflective. The back lens surfaces comprise spherical, hemispherical microlenses, while those on the front surface of the sheeting consist of non-spherical, hemispherical microlenses. The microlenses on one side of the sheeting interact with the microlenses on the other side of the sheeting to produce the desired reflectance characteristics.

US Patent No. 4,576,850 (Martens) relates to molded plastic articles made by replicating a microstructure-bearing surface with a UV-curable organic oligomeric composition.

A number of other patents also relate to molded plastic articles made by replicating a surface bearing a microstructure. See, for example, U.S. Patent Nos. 3,689,346 (Rowland);

4,414,316 (Conley); 4,420,502 (Conley); and 4,373,077 (Kerfield).

US Patent No. 1,824,353 discloses an array of elliptical microlenses for focusing incident collimated light through corresponding openings in a screen or onto a photographic plate with respective applications for rear projection cinema screens and for low light photography.

Summary of the invention

According to a first embodiment of the invention, a transparent carrier film according to the present claim 1 is provided.

According to a second embodiment of the invention, a method for producing a carrier film according to the present claim 14 is provided.

The invention provides a transparent carrier film which can be used to make retroreflective articles, including, for example, transparent protective covers for documents. The covers can be imaged with information to reinforce the authenticity of the document, such as a drawing or photograph of the owner, and also to provide high resistance to tampering. Some of the information can be in the form of so-called "tilting images" which can only be viewed within a narrow angle range and change colour within that angle range. Such images can be viewed under ambient light, thus providing an easily recognisable confirmation of the authenticity of the document. A "tilting image" can also provide a second level of authenticity by becoming glossy when viewed under retroreflective conditions, thus making it particularly difficult to to either manipulate the document or counterfeit the new coating. The invention also provides a method for producing such carrier films.

Typically, a specularly reflective layer is applied to the second side of the carrier film to form a retroreflective article. If the reflective layer is transparent, the retroreflective article can be used as an overlay.

Short description of the drawing

The invention is explained in more detail with reference to the drawing. The drawing shows:

Fig. 1 is a schematic cross-section through a portion of a retroreflective sheeting according to the invention;

Fig. 2 is a schematic cross-section through a portion of an embodiment of the transparent protective cover according to the invention after it has been laminated to the front of a document;

Fig. 3 is a schematic cross-section through a portion of another embodiment of the transparent protective cover according to the invention; and

Fig. 4 is a micrograph of the front side of a carrier film according to the invention covered with microlenses.

Fig. 1-3, which are shown in an idealized manner, are not to scale and are intended to be illustrative and not limiting.

Detailed description of illustrative embodiments

As seen in Figure 1, in a typical retroreflective embodiment, the article 10, e.g., a retroreflective sheeting, comprises a carrier sheet 11 having first and second broad sides, an array 14 of substantially hemispherical microlenses on the first broad side, and a substantially continuous specularly reflective layer 17 on the second side. The carrier sheet 11 is substantially transparent and has the proper refractive index and dimensions so that collimated light orthogonally incident on the microlens array is focused approximately on the second side of the carrier sheet 11, i.e., on the specularly reflective layer 17.

In the carrier films, the second or back side is substantially flat and planar, whereas the first or front side has an array of convex microlenses. The microlenses are substantially hemispherical. A spherical shape is the figure formed when an ellipse is rotated about one of its axes. As used herein, the term "hemispherical" refers to the fact that each microlens is only a portion of a spherical shape. The axis of each microlens is substantially perpendicular to the plane of the carrier film at the base of the microlens. The circumference of each microlens is widest at its base where it protrudes from the carrier film. The microlenses are generally very tiny, typically having a width on the order of about 50 to about 150 m at their base. The base of each microlens is proportionally wider than the base of a true hemisphere of the same height to increase the width of the conical region in which incident light is focused onto the second side of the carrier film. Typically, light shining on the mi Light incident within the microlens array in a conical region oriented perpendicular to the carrier film and having a total width of about 5 to about 10 degrees is focused onto the second side of the carrier film and, in retroreflective embodiments, is reflected back. Light incident within this orientation is referred to herein as "substantially orthogonal."

Preferably, the microlenses are densely packed within the array to improve the degree of focus and, in retroreflective embodiments, to increase the retroreflective brightness. In some embodiments, the microlenses may be so densely packed that they are substantially abutting, i.e., that the adjacent microlenses are in contact rather than spaced apart. In some cases, adjacent microlenses may partially overlap, as shown in Figure 4. An advantage of the invention is that the microlenses are not regularly arranged, i.e., that no special arrangement, for example, a cubic or hexagonal arrangement, is necessary. As shown in Figure 4, the microlenses are arranged in a substantially random arrangement.

If the article is to be used as a document overlay, the reflective layer is preferably transparent. Illustrative examples of reflective layers are those of zinc sulfide, cryolite, lithium fluoride, silicon oxides, and magnesium fluoride. U.S. Patent No. 3,700,305 (Bingham et al.) and the above-mentioned U.S. Patent No. 3,801,183, which are incorporated herein by reference, disclose the use of dielectric mirrors as partially light-transmitting retroreflectors. If the reflective layer is transparent, it can be continuous without obscuring information on the front of a document to which the new protective overlay is applied. However, the reflective layer can be opaque, if it is interrupted and applied in such a way that it does not cover any information on the front of the document to which it is applied.

A pattern of image-forming material, such as clear ink or clear varnish, may be applied to the second side of the carrier film before a dielectric mirror is applied thereto to provide an inscription or inscriptions within a coating according to the invention.

Instead of, or in addition to, a clear ink message or pattern, the second side of the transparent polymer layer (under the reflective layer) may be printed with an opaque ink. For example, if the overlay is to be used to protect a document that has previously been printed with information that is standard on all similar documents, such as boxes designated for inserting the owner's name, address, and the like, those boxes can be filled with owner-specific information by printing in opaque ink. If someone were able to peel such an overlay off a document, they would remove at least part of the opaque ink layer. Then, to change the information, one would have to erase any part of that layer left on either the document or the overlay and insert fake information to fill the boxes. Accomplishing this task would be particularly difficult if part of the ink layer was transparent, forming a so-called "tilt image" in a pattern that would be easily destroyed if the coating was tampered with.

To create a "tilting image", a message or pattern is printed in clear ink on the second side of the carrier film before a reflective layer is applied to the se is applied. The resulting change in the surface finish of the carrier film usually results in changes in the thickness of the reflective layer as it is applied, creating an image that can be repeated across the entire front of the overlay and can be seen by the naked eye within a narrow range of angles under ambient light. The color of the image changes at different viewing angles and is therefore sometimes referred to as a "tilt image." An attempt to peel off the overlay to change the underlying information is likely to destroy the colored "tilt image" and thus attract the attention of anyone viewing the document. Since the colored "tilt image" shines brightly when viewed under retroreflective conditions, a retroreflective examination can provide certain proof that the document is authentic.

Fig. 2 illustrates another embodiment in which a coating 20 comprises a carrier film 21 consisting of two layers, i.e., a transparent spacer layer 22 and an array 24 of microlenses. An advantage of such multilayer embodiments is that the materials used for the spacer layer 22 and the lens array 24 can be independently selected to optimize the overall properties of the article. For example, a high performance molding material used to make the random array may be relatively expensive, so a cheaper material is used for the spacer layer to reduce the overall cost of the article. In another example, in a document coating, the random pattern may be chosen to be very hard and abrasion resistant, providing greater durability, whereas the spacer layer may be chosen to have a relatively low tensile strength, making it more difficult to remove intact from a document.

Referring again to Fig. 2, on the back side of the spacer layer 22, i.e., the side opposite the random array 24, there is an intermittent layer 26 of clear ink, on which there is a continuous transparent reflective layer 27. The reflective layer 27 is covered by an adhesive layer 28 which can be used to attach the overlay 20 to the front side of the document 29 over the information 29A printed thereon.

Because the retroreflective material 27 adhered better to the areas containing clear ink 26 than to the non-ink areas of the spacer layer 22 when applied, e.g., by vacuum sputtering, it is thicker in the ink-containing areas. The resulting variations in the thickness of the clear reflective layer 27 create a colored "tilt image" that can be seen through the polymeric layer under ambient light, which image shines brightly when viewed under retroreflective conditions.

A wide variety of adhesives can be used on articles of the invention, including, for example, hot melt adhesives, pressure sensitive adhesives, and actinically activated adhesives, such as heat activated and UV activated adhesives. For coatings on documents, the adhesive should be substantially transparent.

In some embodiments, document overlays can be made with an adhesive layer disposed between the second side of the carrier film and the reflective layer and on the side of the carrier film opposite the reflective layer. Fig. 3 shows such an embodiment in which an overlay 30 comprises a transparent carrier film 31 consisting of a spacer layer 32, a random arrangement of microlenses 34 and a transparent adhesive layer 33. In such In some embodiments, light falling on the assembly 34 is focused onto the back of the adhesive layer 33, i.e., the side opposite the spacer layer 32. On the back of the adhesive 33, i.e., the side opposite the carrier film 31, there is applied an ink pattern 36 onto which has been applied a transparent reflective layer 37 which adheres to the ink 36 but not to the adhesive 33. The reflective layer 37 is covered by a further adhesive layer 38 with which the overlay 30 can be bonded to the front of a document (not shown). If the adhesive layers 33 and 38 have an identical composition, an attempt to remove the overlay 30 from a document is likely to tear the two adhesive layers apart and thereby make it extremely difficult for a person to tamper with the document and then reapply the overlay without leaving a clear sign of tampering.

An advantage of carrier films according to the present invention is that they can be used to produce retroreflective articles without using a monolayer of relatively expensive microlenses. Retroreflective articles according to the invention have, as mentioned above, a selective angular character and can be produced relatively inexpensively. In the case of document coatings, all information and patterns printed between the back of the carrier film and the front of the document are protected by the carrier film. In order to achieve very good abrasion and weather resistance, resins can be selected which are used to form the carrier film and in particular the random arrangement of microlenses.

A carrier film composite which can be converted into a retroreflective film according to the invention can be prepared by first forming a retroreflective film as a starting form lie comprising a monolayer of encapsulated glass microspheres and a spacer layer carrying a metallic coating, e.g. the specularly reflective layer of Fig. 4 in U.S. Patent No. 4,505,967 (Bailey). Such a metallic coating has a surface with hemispherical projections on which a master mold can be formed by the following process:

a) preparing a transparent, curable composition, for example a UV-curable oligomeric composition, such as that disclosed in U.S. Patent No. 4,576,850;

b) applying this oligomeric composition to the fully hardened surface of the initial mold;

c) applying a strand of the composition between a carrier film and the preform, the carrier film being substantially planar and at least one of the carrier film and the preform being flexible, as taught in U.S. Patent No. 4,374,077;

d) curing the applied composition, e.g. by UV radiation, to obtain a composite of the carrier film and the cured oligomeric composition; and

e) removing the composite from the initial mould to obtain a master mould with a surface in which hemispherical cavities are present.

In the above-mentioned US Patent No. 4,374,077, a replication by polymerization of a liquid mass is disclosed, which is applied between a shaped surface and a carrier film to which the mass is to adhere.

The composite with its surface having hemispherical cavities can be used as a master from which a carrier film according to the invention is made. This can be done by carrying out a similar process using the master so made and a carrier film or base film of suitable thickness to act as a spacer layer. Illustrative examples of suitable base films are flexible films of polyester, polyvinyl chloride and polymethyl methacrylate. Alternatively, the master can be specially machined to form a desired random array of microlenses. Typically, it is somewhat expensive to make masters in this way, but an advantage of the process described above is that a suitable master can be made in a fairly economical manner.

The curable composition is chosen in part for its processability, i.e., its viscosity, its curing conditions, etc., and its post-curing properties, i.e., its abrasion resistance, clarity, etc. The illustrative class of curable compositions include the UV radiation curable compositions disclosed in the above-mentioned U.S. Patent No. 4,576,850. This patent discloses one-part, solvent-free, radiation addition polymerizable oligomeric compositions containing hard and soft segments. The compositions typically contain between 0.1 and about 0.5 weight percent of a photoinitiator. The hard segments are preferably polyurethane, and the soft segments are preferably polyester.

The curable composition is preferably applied to the master mold in a limited amount so that, for example, the composition completely fills the cavities but does not protrude beyond the surface of the master mold by more than about 20 percent of the average depth of the hemispherical cavity. cavities. In this way, the carrier film, which becomes at least a portion of the spacer layer, can be used to substantially determine the desired overall tensile and film-like stability of the article. Also, the curable composition is typically more expensive than the carrier film material, and it is typically easier to control shrinkage and warpage upon curing when the curable composition has a smaller thickness. Preferably, the random array of microlenses is between about 0.01 and about 0.05 millimeters thick when a retroreflective sheeting containing a monolayer of microspheres having an average diameter of about 65 micrometers is used as the starting form as described above. In such cases, the spacer layer is typically between about 0.2 and about 0.3 millimeters thick.

The viscosity of the oligomeric composition applied in step b) is preferably in the range of about 1000 to about 5000 centipoise ("cps"). Above this range, air bubbles may be trapped and the resin may not completely fill the cavities in the molding surface of the master mold. Below this range, the resin may shrink so much during curing that the cured resin may not completely replicate the molding surface of the master mold. Preferably, the viscosity of the resin is between about 2000 and about 3000 cps. Within this preferred range, the oligomeric composition should completely fill the cavities in the master mold without the need to apply pressure. However, if the cavities are unusually deep and/or narrow, it may be desirable to reduce the viscosity to below 2000 cps because some shrinkage is preferable to failure in order to fully meet the characteristics.

In order to achieve the desired viscosity in compositions such as those disclosed in U.S. Patent No. 4,576,850, it may be necessary to include in the oligomeric composition an ethylenically unsaturated monomer, for example an alkyl acrylate, preferably an alkyl acrylate in which the alkyl group contains a straight chain of 4 to 12 carbon atoms. The relative amounts of oligomer and monomer should be controlled so that the total equivalent weight of the composition does not become so low as to cause unacceptably high shrinkage upon curing.

Polyvinyl chloride is a preferred substrate for use in step c) because it is typically economical, durable, transparent, and dimensionally stable. Other illustrative examples of suitable carrier films are films of polycarbonate, biaxially oriented polypropylene, and biaxially oriented polyethylene terephthalate whose surfaces have been treated, e.g., by corona treatment, to promote strong adhesion to the oligomeric composition. Each of these substrates, typically used in thicknesses between about 0.05 and about 0.4 millimeters, can be bent back and forth to spread a resin strand, as taught in U.S. Patent No. 4,374,077. Other illustrative examples of suitable carrier materials are films of cellulose acetate butyrate, cellulose acetate propionate, polyethersulfone, polymethyl methacrylate, and polyurethane, as well as glass.

The curable composition and the carrier film are preferably selected so that a strong bond is formed between them during curing.

The optimal thickness of the carrier layer or spacer layer depends in part on the average size and shape of the microlenses. In a typical retroreflective film lie, for example, as used as a starting form in the manner described above, 80 percent of the microspheres have a diameter in the range of 15 percent of the average. If the average diameter of the microspheres is about 65 m, the microlenses in the resulting carrier film of the invention have an approximate average focal length between about 0.2 and about 0.3 millimeters and thereby a spacer layer can be between about 0.2 and about 0.3 millimeters thick or somewhat thinner if a transparent adhesive layer is provided between the carrier film and the reflective layer. Since a thinner carrier film would be more economical and equally suitable, it is preferable to start with a retroreflective sheeting having glass microspheres of smaller average diameter.

Articles according to the invention can be manufactured in a substantially rigid or in a flexible form. For example, with the selection of suitable materials, articles according to the invention can be flexible enough to be wound around a 1 cm diameter mandrel into a roll form.

Examples

The invention is further explained by the following illustrative examples, which are not intended to be limiting. Unless otherwise indicated, all amounts are expressed as parts by weight.

Reflection brightness

The retroluminosity was measured at a divergence angle of 0.2º and an entrance angle of -4º in candelas per lumen using a retroluminometer as described in U.S. Department of Defense publication T 987,003.

example 1

A one-part, solvent-free, radiation-addition polymerizable, crosslinkable, organic oligomeric composition having hard segments and soft segments and a viscosity of 1600 cps was prepared by mixing the following ingredients:

Parts

Acrylate-capped polycaprolactone urethane oligomer 54.3

N-vinylpyrrolidone 16.3

[(Ethoxy)-2-ethoxy]ethyl acrylate 11.3

1,6-Hexanediol diacrylate 5,7

N-(Isobutoxymethyl)acrylamide 11.1

Mixture of tertiary amines (TINUVIN 292 from Ciby-Geigy) 1.0

1-Hydroxycyclohexylacetophenone 0.25

The surface of the initial mold used was the metallic reflective layer of a lens-enclosed retroreflective sheeting comprising a monolayer of glass microspheres having an average diameter of about 65 microns. It should be noted that true spheres are not within the scope of the present invention. The liquid oligomeric composition was applied to the metallic layer in an amount just sufficient to fill the cavities of the metallic layer. On top of this was applied a clear polycarbonate film having a thickness of 0.125 millimeters through which the oligomeric composition was irradiated in 2 passes under a medium pressure mercury lamp of 350 to 380 nanometers at 125 W per linear centimeter, thus giving a total exposure of 110 mJ/cm2. The contoured surface of the resulting first composite consisted essentially of abutting hemispherical concave shapes.

The same process was repeated using the resulting composite as a master mold, thus forming a second composite having a total thickness of about 0.2 millimeters and consisting of a polycarbonate carrier film and an array of substantially hemispherical microlenses.

Aluminum was vacuum deposited onto the flat surface of the second composite to a thickness of about 70 nanometers. The resulting retroreflective film had a retroreflective brightness of 390 candelas per lumen.

Example 2

A composite was prepared like the second composite of Example 1, except that the thickness of its polycarbonate support was 0.25 millimeters. The photomicrograph in Fig. 4 shows the resulting array of microlenses.

A pattern of clear ink (RAGE-800 ink from Advance Process Supply in Chicago) was screen printed onto the flat surface of the polycarbonate carrier film, drying to a thickness of less than 0.01 millimeters. Zinc sulfide was then vacuum deposited to a thickness of about a quarter wavelength of 520 nanometers to create a clear reflective layer. The ZnS tended to adhere only to the clear ink and the areas immediately surrounding the ink layers, leaving the exposed polycarbonate areas uncovered. A 0.05 millimeter thick layer of ELVAX 550, a DuPont ethylene-vinyl acetate copolymer hot melt adhesive, was then laminated on top. The The resulting transparent protective coating according to the drawing was laminated with its adhesive layer at 82ºC to the front of a driving license which had printed information on it.

When viewed through the overlay, this printed information was clearly legible despite a slightly colored "tilt" where the clear ink had been applied, and this image was a pattern that extended across the entire front of the license and was clearly visible under ambient light. When viewed under specular reflection, the pattern was brilliantly bright.

The microlenses were scrubbed with fine sandpaper to simulate the wear that would be expected from normal use over a period of several years, and after that the printed information remained legible and the colored "tilt image" remained clearly visible under ambient light.

Example 3

A driving license was produced in the same way as in Example 1, only with the following differences:

a) the carrier consisted of poly(vinyl chloride);

b) two layers of adhesive were applied as shown in Fig. 3;

c) a pattern of the colourless ink was screen printed onto the exposed side of the adhesive layer applied to the second side of the carrier film;

d) ZnS was applied to this sample and only stuck to the ink, not to the adhesive.

When this license was viewed through the overlay, a "tilt image" similar to that of Example 2 was visible under ambient light and was brilliantly bright under specular reflection.

Example 4

A composite (approximately 15 x 28 centimeters) was prepared like the second composite of Example 1, except that the carrier film was made of polyvinyl chloride 0.25 millimeters thick and the composite was converted into a retroreflective film by vacuum deposition of ZnS to a thickness of about a quarter wavelength of 520 nanometers. Measurements of retroreflective brightness were made at each of the four corners and at the midpoints along the long sides. The results in candelas per lumen were:

68 78 71

56 71 69.

Given that the thermoplastic film of polyvinyl chloride inevitably has thickness variations that result in a somewhat unevenly thick spacer layer and that the microlenses were of different sizes, these values are remarkably consistent.

Example 5

A composite was prepared as in Example 2, except that the colorless ink was replaced with a colored, transparent ink having the following formulation: 80 parts colorless ink (Roto/Flexo Extender FA-1626 from Sinclair and Valentine, St. Paul, Minnesota) and 20 parts interference pigment AFFLAIR 235 Rutile Green Pearl from EM Industries, Inc., Hawthorne, New York. The composite was then processed as in Example 2. Example 2 was converted into a retroreflective film that was stuck to the front of a driver's license. The resulting "tilted image" was more colored than that obtained in Example 2, but the information printed on the front of the driver's license was readable through the image.

Various modifications and variations of the invention will become apparent to those skilled in the art without departing from the scope of the invention, which is limited to the scope defined in the appended claims.

Claims (19)

1. Transparent carrier film (11, 21, 31) having a first and a second broad side, the second side being substantially flat, and the first side comprising an array of substantially hemispherical microlenses, the microlenses being wider at their base than true hemispheres of the same height, and the shape of the microlenses and the thickness of the carrier film (11, 21, 31) being such that collimated light falling substantially orthogonally on the first side is focused approximately on the second side; characterized in that the array of microlenses is formed by a random arrangement (14, 24, 34) of the substantially hemispherical microlenses of different heights. 2. Carrier film (11, 21, 31) according to claim 1, in which the microlenses substantially abut one another. 3. Carrier film according to claim 1 or 2, wherein the carrier film (21, 31) comprises a transparent spacer layer (22, 32) with a substantially planar first and second broad side and a microlens layer comprising the arrangement (24, 34) of microlenses, the microlens layer being bonded to the first broad side of the spacer layer (22, 32). 4. A retroreflective article (10, 20, 30) comprising the carrier film (11, 21, 31) of claim 1 or 2, wherein the second side of the carrier film (11, 21, 31) has a specularly reflective layer (17, 27, 37) such that collimated light incident substantially orthogonally on the microlens array (14, 24, 34) is directed approximately onto the two th side so that it is reflected back by the object (10, 20, 30). 5. Article (30) according to claim 4, wherein the reflective layer (37) is interrupted. 6. Article (30, 40) according to claim 4, wherein the supporting film (21, 31) comprises a transparent spacer layer (22, 32) having a substantially planar first and second broad side and a microlens layer comprising the array (24, 34) of microlenses, the spacer layer (22, 32) and the microlens layer being bonded together, and the spacer layer being arranged between the microlens layer and the reflective layer (27, 37). 7. Article (20, 30) according to claim 6, wherein a) the total thickness of the object (20, 30) is between 0.05 and 0.4 millimetres; and/or b) the total thickness of the spacer layer (22, 32) is between 0.2 and 0.3 millimetres. 8. The article (20, 30) of claim 6, wherein the article (20, 30) further comprises an adhesive layer (28, 38) on the side of the reflective layer (27, 37) opposite the spacer layer (22, 32). 9. Article (20, 30) according to claim 8, wherein the reflective layer (27, 37) and the adhesive layer (28, 38) are transparent. 10. Article (20, 30) according to claim 9, wherein the article (20, 30) further comprises an image-forming material (26, 36) disposed between the second side of the carrier film (21, 31) and the reflective layer (27, 37). 11. The article (20) of claim 10, wherein at least a portion of the image-forming material (26) is clear and forms an image that, when the article (20) is adhered to a document (29) having information (29A) thereon using the adhesive layer (28), can be seen with the naked eye under ambient lighting without affecting the readability of the information (29A) on the document (29). 12. Article (30) according to claim 4, wherein the reflective layer (37) is opaque and discontinuous. 13. Article (30) according to claim 4, in which the reflective layer (27) of the article (20) is bonded to an underlying document (29) via an adhesive layer (28). 14. A method for producing a carrier film (11, 21, 31), the method comprising the following steps: a) preparing a curable composition; b) applying the composition to the surface of a master mold having a random arrangement of substantially hemispherical cavities of varying depth, the cavities having a wider opening than true hemispherical cavities of the same depth; c) applying a strand of the composition between a transparent carrier film and the master mold, wherein the carrier film is substantially planar and at least one of the master mold and the carrier film is flexible; d) curing the applied composition to obtain a composite material with a random arrangement (14, 24, 34) of essentially hemispherical microlenses of different heights bonded to the carrier film; and e) removing the composite from the master mold to obtain the carrier film, the shape of the microlenses and the thickness of the carrier film (11, 21, 31) being such that collimated light incident substantially orthogonally on the array (14, 24, 34) of microlenses on the first side of the carrier film is focused approximately on the substantially flat second side of the film. 15. The method of claim 14, wherein the curable composition is a UV-curable, one-part, solvent-free, radiation addition-polymerizable, crosslinkable, organic oligomeric composition containing hard segments and soft segments, and wherein UV radiation is used to cure. 16. The method of claim 15, wherein the hard segments are polyurethane and the soft segments are polyester. 17. The method of claim 14, wherein the composition does not extend beyond the surface of the master mold more than 20 percent of the average depth of the hemispherical cavities. 18. The method of claim 14 or 17, wherein the method further comprises applying a specularly reflective layer (17, 27, 37) to the exposed surface of the carrier film to form a retroreflective film (10, 20, 30). 19. Method according to claim 14, in which the master mold is produced with the following steps: a) providing a master mold comprising a retroreflective sheeting, the sheeting comprising a monolayer of randomly arranged microspheres and a spacer layer carrying a metallic coating having hemispherical projections, the projections being wider at their base than true hemispherical projections of the same height; b) preparing a second curable composition; c) applying the second composition to the metallic coating; d) applying a strand of the second composition between a second carrier film and the master mold, wherein the second carrier film is substantially planar and at least one of the master mold and the second carrier film is flexible; e) curing the second composition to form a second composite comprising the second carrier film and the second composition; f) Removing the second composite from the master mold to obtain the master mold.